1. Field of Invention
The present invention relates to a code implantation process of a mask read only memory (MROM). More particularly, the present invention relates to a self-aligned code implantation process of a mask read only memory (MROM).
2. Description of Related Art
In general, the mask read only memory (MROM) structure includes a plurality of bitlines and a plurality of polysilicon wordlines bridging over the bitlines. Channel regions of the memory cells are beneath the wordlines and between two neighboring bitlines. The MROM cells can be programmed to store two-bit data. For the MROM cell programming, the stored data is either “0” or “1” depending on whether the dopant ions are implanted into the channel regions or not. Such implantation process, implanting dopants into the specific channel regions, is so called code implantation process.
Ordinarily, the code implantation process of the MROM includes the following steps. Using a photomask, the resist layer over the substrate is patterned to expose the channel regions that are to be coded. Next, using the patterned resist layer as a mask, an ion implantation process is performed to dope ions into the predetermined channel regions. However, the patterned resist layer (code mask layer) usually includes an isolated pattern region and a dense pattern region. During the exposure step, the dense pattern region can easily have erroneous critical dimensions (CDs) due to the optical proximity effect. Therefore, misalignment can occur if the patterned resist layer with erroneous CDs is used as a mask for code implantation, leading to faulty data storage in the memory cells. As a result, it deteriorates the operation and reliability of the memory device.
In the prior art optical proximity correction (OPC) or/and phase shift mask technologies are usually applied for helping control the critical dimensions of the openings in the dense pattern region. For OPC, it is required to design special patterns in the masks to correct the erroneous CDs resulting from optical proximity effect, which is very time-consuming and thus increase the costs. Moreover, it is very difficult to debug the defeats in the mask patterns.
Furthermore, if the 248 nm-exposure system is applied in the photolithography process, the exposure resolution has a limitation of about 0.16-0.18 critical dimensions. In order to improve the exposure resolution, light of a wavelength at 198 nm is needed for the lithography process. However, not only the exposure system of 198 nm is high-priced but the compatible photoresist materials are also more expensive.
In addition, misalignment resulting from erroneous CDs of the code mask layer can cause out-diffusion of the doped ions (code ions) in the channel regions. The code ions implanted by the code implantation may diffuse into the buried bitlines, thus changing the dopant concentrations and the current of the buried bitlines.